1. Field of the Invention
The present invention is related to a method and an apparatus for detecting yarn tension and a method for winding a yarn.
2. Description of Related Art
As is well known, in a winding process of a synthetic fiber yarn, a desired value of yarn tension is varied in accordance with winding conditions, such as a thickness of the yarn, a feed speed of the yarn, and a kind of the yarn etc.
When the winding tension is excessively high, a so-called bulging phenomenon is generated, wherein a completed yarn package is subjected to a bulging at its side portions, which is likely to be subjected to rubbing during the transportation of the package in a container, which, during a subsequent package treating process, makes it difficult to take out the yarn smoothly. Furthermore, a so-called saddling phenomenon is generated, wherein the package is, at its central part of the periphery, depressed, which makes the yarn quality different between the central portion and end portion at the outer periphery of the package, which cause these parts to be dyed differently at a later stage. Furthermore, the excessively high tension causes an inner stress in the package to be increased, so that a so-called spiraling may be generated, where a portion of a yarn layer is displaced over the remaining portion due to a local relaxation of the yarn tension, resulting in worsening the unwinding performance of the package. Furthermore, a bobbin as a core of the package is subjected to an increased degree of a deformation, which make it difficult for the completed (full) package be easily detached from the corresponding spindle of the winding machine.
Contrary to this, an excessively small tension causes the movement of the yarn to be unstable, so that filaments constructing the yarn are apt to be separated or looped at a guide for guiding the movement of the yarn, which may cause the quality of the yarn to be reduced. Furthermore, the density of the package may be reduced, which makes it difficult for the package to keep a desired shape, so that the shape of the package is deformed during the execution of the winding process or during transportation of the package to the following treating process, which makes the package unusable. Furthermore, a constraint on the yarn during the execution of winding is not sufficiently strong, so that a so-called yarn shift phenomenon may be generated, by which a part of the yarn in an intermediate layer of a package is displaced toward the center of the package, which makes it difficult for the yarn part to be unwound.
In order to prevent above mentioned problems from occurring, it was heretofore been usual to execute a test winding process where a number of test windings are done by changing yarn taking up speed to determine a suitable taking up speed, i.e., a suitable yarn tension to obtain a desired shape of a completed package.
However, the thus determined yarn taking up speed can not permanently maintain the desired winding condition due to the fact that a yarn tension during the winding is made different from the desired yarn tension due to inevitable changes in winding condition such as a change in a room temperature, a small variation in a viscosity of a polymer material, a change in a process temperature and an abrasion of any yarn guiding member.
In view of the above, it is necessary, during the execution of the winding process, that monitoring of any change in a winding condition is done and that, based on the result of the monitoring, process control, such as modification of a winding condition or spinning condition or an operating condition of the winding system and a determination of a degree of a quality of the completed package, must be done.
Heretofore, two methods have been known for executing process control in a yarn winding process. The one is based on a measurement of a shape as well as a winding density of the completed package. The other is based on a continuous or periodical measurement of a yarn tension during the execution of the winding.
The first method where the yarn shape as well as winding density are measured is defective in a delayed detection of the fact that the winding is not suitable due to the fact that the detection of an undesired shape or winding density is possible only after the package formation is completed. The first method is also disadvantageous in that a temporally generated defect is "hidden" in the completed package, i.e., can not be found by merely inspecting an outer appearance of the completed package. Thus, it may be possible that a package including a hidden defect may be used at a following process such as a knitting. Furthermore, an abnormally increased tension, which may occur very rapidly, may inevitably cause the finished package to be completely damaged.
In the second idea based on a continuous or periodical detection of the yarn tension during an execution of a winding condition, a method has also been practiced, where the yarn tension is monitored continuously and a positive control of the taking up speed is done so that it is controlled predetermined constant value.
In order to detect the yarn tension, a so-called three point type tension measuring device having spaced fixed yarn guides 61 and 62, a movable guide 63 arranged between the guides 61 and 62 and a displacement sensor (strain gauge) 64 for detecting a movement (deflection), as shown in FIG. 27 has heretofore been usually used.
In this type of a tension measurement device, a detecting signal does not detect an absolute value of the yarn tension. Thus, a calibration of the detected value is essential. It is ideal that such a calibration is done with respect to a yarn subjected to a movement at a speed corresponding a speed of a yarn during a winding such as a value larger than 3,000 m/min. However, from the practical view point, it is difficult to execute a calibration while the yarn is moved under such a high speed condition. Thus, generally, yarn tension is measured in a stationary yarn while being weighted in a range corresponding to a measuring range of tension. Then, a calibration is done in such a way that the measured value corresponds to the weight value.
However, it is quite often that the tension detecting device providing precise detected value during a stable state provides a different detected value when the yarn is moving. It is considered that such a difference of a measured value is generated by a small difference in various factors having different degree of influence to the measured value, such as a coefficient of friction of a guide due to a difference of a surface condition, a shape, a detecting position, and a thickness of the yarn, In other words, even if a tension measuring device of the same operating principle is used, the measured tension value may be different due to the difference in it precision, i.e., there is no compatibility between the measuring devices. Furthermore, the measured value is also influenced by a abrasion of a yarn guide caused by an aging.
In view of the above, in the prior art three point tension detector, there is lacking a compatibility in the detected (displayed) value. In other words, the optimum tension value determined at an actual winding test is a value intrinsic to the particular measurement system. Thus, the optimum tension value for a different detector is unknown. In other words, there is inevitable difference in the optimum tension value between the tension measuring devices. On the other hand, from the practical view point, a use of a plurality of tension measuring devices is inevitable in a factory for obtaining a high precision control of the winding state across the whole factory for a prolonged time. Thus, in view of the variation in detected valued between the devices and a change in the detected value due to aging, a precise evaluation of a defect can be done only when the defect is of a large degree. Thus, it is impossible that a package having a smaller defect is prevented from being fed to a subsequent process.
Furthermore, in the prior art method for controlling a yarn tension, if the detected values of the tensions of yarns are the same, values of real tension of the yarns being subjected to a winding may not be the same. Thus, the finished packages can no, be controlled to the same shape irrespective of a fact that the detected tension is the same.
In another prior art, a tension detecting device of a contact type of so-called FTS type is known, where a movable guide is arranged in a yarn path, so that a movement (deformation) of the movable guide is detected when the yarn during a traverse movement is made to contact the guide, so that a signal of the movement of the guide as an indicative of on a yarn tension is detected by a strain gauge.
This type of device for measurement of the yarn tension is advantageous over the three point type in FIG. 27 in having a smaller resistance force at the guide. However, a speed of a yarn higher than 3,000 m/min may also cause filament breakage or loop to be generated due to a change in the quality of the same yarn.
Furthermore, in both of the three point type and the FTS type, an adjustment is required periodically. In these sensors, the measured value of the movement of a movable guide is a value which is transformed into a value of the tension. Thus, a large difference between the yarn speed during the adjustment of the scale and the yarn speed during the measurement causes the error to increase. Furthermore, individual differences are also large.
In view of the above, Japanese Unexamined Patent Publication No. 59-88654 discloses a method where a yarn is subjected to vibration by means of an ultrasonic wave and a change in the condition of the vibration is detected by a detector, so that a yarn tension is known. In this method, a deviation of a vibration from a resonance point, which is determined by a yarn tension, is measured. Furthermore, by searching for a resonant frequency, a resonance point is found, which allows the yarn tension to be detected.
However, the above method for detecting a yarn tension by using a deviation of the vibration of the yarn by the ultrasonic wave is defective in that a large amount of deviation causes a change to be reduced in an amplitude due to the lack in a proportional relationship between the deviation and the amplitude, which makes it difficult to measure a yarn tension. Furthermore, during measurement of the change in the amplitude, the detected value varies in accordance with various conditions, such as contact condition between the ultrasonic detector and the yarn and a change in sensitivity of the ultrasonic detector, which necessitates a calibration at each of the detecting locations.
Furthermore, there exists a plurality of resonance frequencies, that are a first, second, . . . , n-th order of resonance frequencies, in the method for detecting a resonant point by seeking the resonance frequency, it is difficult to decide the order of the resonance merely from the amplitude of the frequency, which makes it difficult to determine the tension.